Embodiments of the present invention relate generally to structures, particularly but not exclusively subsea structures, and more specifically, to a structure monitoring system and method.
Subsea production risers typically include several kilometer long kilometer-long pipes linking seabed equipment with surface production facilities. As the subsea installation is located deeper and deeper, there is a growing need for monitoring the dynamic behavior of the pipes connected to it. Monitoring strain of the pipes is a way to assess their operating conditions. In typical operating conditions, risers are subject to very complex loads, including self-weight, internal pressure from the production fluid, static deformation caused by the surface facility, tension from buoyancy modules attached to the structure, Vortex Induced Vibrations, and installation related stresses when the risers are being deployed. These loads generate mechanical stresses on the riser pipes. Risers are designed according to a certain set of load scenarios, in order to operate reliably in real conditions.
One critical aspect of riser engineering is its dynamic behavior during operation, during which it is subjected to loads changing with time. The typical resonance frequency range for a vertical rigid riser is in the range of 10-3-10 Hz. Typically, 100 frequency modes are of interest for a subsea riser pipe. Various systems have been developed or proposed for the monitoring of risers' mechanical behavior in the field. One is based on the monitoring of riser movement using an accelerometer platform. Another approach is the direct measurement of the strain within the riser. One proposed approach is to use optical fibers as strain gauges. There are several ways of measuring strain in optical fibers. Strain measurement can either be localized, semi-distributed or distributed depending on the technology used for the interrogation of the fibers. Distributed strain measurement along an optical fiber can be done through Brillouin scattering. Brillouin measurements can also provide distributed temperature measurements. Brillouin based measurements can be made along few 10s of km of optical fiber. However, Brillouin measurement also has limitations. Its spatial resolution is in the range of a few meters and measurement time can be in the range of a few minutes. Therefore, it is not very suitable for measurement of dynamic strain above a few 10s of MilliHertz. This is a significant limitation, considering the typical resonance frequency range of a riser pipe.
Another approach for measurement of strain on riser pipes includes utilizing fiber Bragg gratings (FBGs) that are distributed along a length of the optical fiber which covers the length of the riser pipe between the surface facility and the touchdown point of the riser pipe. However, the number of frequency modes that need to be studied and the length of riser pipes leads to an increase in the number of gratings that need to be defined on the fiber to measure stresses acting on the length of the riser pipe. Further, in this approach, one fiber is utilized to study one particular type of stress (elongation or contraction along the FBG) acting on the riser pipe. Hence, to analyze the stress state of the riser, one would require multiple fibers coupled to the riser pipe at different angular positions.
The number of gratings and fiber cables required to study all stresses acting on the pipe make current approaches expensive. Further, multiple fibers and gratings make the installation process complex and add to installation costs. Further, the dependence on multiple gratings installed in parallel on the pipe also adds to the inaccuracy that may creep into measurements.
Hence, there is a need for a method and system that utilizes optimal gratings to study stress and a single fiber to cover the riser pipe with these gratings.